Typical asphalt road paving machines (or asphalt pavers) have a hopper for receiving hot asphalt paving material located at the front and a conveyor for delivering the asphalt paving material from the hopper to the rear of the machine and depositing the asphalt at the back of the paver onto the road surface to be paved. An auger is located at the rear of the paver for distributing some of the asphalt material laterally to the sides of the road surface. The asphalt paver also includes a floating screed, which is pulled behind the auger, for smoothing out and compressing the asphalt to the desired road mat thickness. The screed unit is pulled behind the auger by a pair of tow arms, which have forward ends pivotally mounted to the sides of asphalt paver and rearward ends pivotally mounted to the screed.
Many modern screeds are provided with a pair of extendible screed units attached to a main screed unit. Each extendible screed unit is movable laterally to one side of the main screed unit in order to vary the width the road mat being laid down by the paver. Some of these extendible screed units are mounted behind the main screed unit, such as the ones shown in U.S. Pat. No. 5,203,642 issued Apr. 20, 1993 to John W. Heller et al. However, front mounted extendible screed units have become popular in recent years due to certain advantages they afford over rear mounted extendible screed units. One such screed with front mounted screed extension units is shown in U.S. Pat. No. 4,379,653, issued Apr. 12, 1983 to Robert L. Brown. Such patent also discloses the use of a strike-off plate positioned in front of the extension units to limit the amount of paving material between the side extension units during retraction.
Screeds are provided with screw adjustments for controlling the "pitch" or "angle of attack" of the screed surface relative to the desired final surface of the paving mat for controlling the depth of the mat being laid by the screed. For instance, if the angle of attack is increased by raising the leading edge of the screed sole plate, the depth of the mat formed by the screed is likewise increased. However as the angle of attack is increased, the height of the strike-off plate of prior screeds also increased. Because such strike-off plate is mounted at a substantial distance ahead of the main screed in front of the screed extensions, the rise in height of the strike-off plate is disproportionately greater than the increase in the mat height being formed by the screed. This results in too much asphalt material being fed to the main screed and an undesirable build-up between the screed extensions. As a consequence, the quantity of asphalt material becomes trapped between the screed extensions, which may prevent the full retraction of the screed extensions when a reduction in mat width is desired during paving operations. If the screed extensions cannot be retracted, the paver must be stopped and the excess asphalt material removed manually with shovels. This is not only time consuming and labor intensive, the stopping of the paver also creates undesirable inconsistencies and discontinuities in the compression of or other imperfections in the asphalt mat in as much as the floatation of the screed is affected by the speed of its forward movement.